Replace solr.xml initContainer with a subPath configMap volumeMount.

**Is your feature request related to a problem? Please describe.**
Currently we use a initContainer to copy over the solr.xml from the configMap and place it in the solr data directory. This is because mounting the entire configMap would wipe out all other data in the directory. However instead of using the initContainer, we can merely use a volumeMount with subPath: `solr.xml`. This will make sure that only the `solr.xml` file is mounted within the directory.

An issue with subPaths and ConfigMap mounts is that pods are not automatically updated when the configMap is updated. However StatefulSets aren't updated on ConfigMap changes in the first place, and #158 provided a method on updating pods on a configMap update.

This should be an easy win to make our statefulSet less complex.

Find the Solr StatefulSet definition and inspect how the solr.xml ConfigMap and initContainer are currently wired. Replace the copy step with a solr.xml subPath volumeMount while preserving the other data in the directory; done means the initContainer is no longer needed and the StatefulSet still mounts the expected file.

Written by the indexing model from the issue text.